1. Technical Field
This invention relates to digital signal decoding for the purposes primarily of audio reproduction. In particular, the invention relates to enhanced synthesis sub-band filtering during decoding of digital audio signals.
2. Description of the Related Art
In order to store or transmit data representing audio signals it is often desirable to first encode or compress the data so as to enable it to be stored or transmitted more efficiently. Decoding the data requires that the stored or transmitted data be reconstructed into audio signals by application of a decoding or decompression technique. The reconstruction process is typically quite computationally intensive, yet the process should be fast and reliable enough to enable the audio signals to be reconstructed in real time, on the fly, for example. In order for the decoding process to be carried out in relatively low-cost consumer products, the hardware utilised by the decoder should also preferably be relatively simple and inexpensive, or at least to the greatest extent reasonably possible.
Efficient stereo and multichannel digital audio signal coding methods have been developed for storage or transmission applications such as Digital Audio Broadcasting (DAB), Integrated Service Digital Network (ISDN), High Definition Television (HDTV) and Set Top Box (STB) for video-on-demand. The formats used to encode and reciprocally decode digital audio and video information for storage and retrieval is subject to various standards, one of which has been established by the Moving Pictures Experts Group and is known as the MPEG standard.
A standard on low bit rate coding for mono or stereo audio signals was established by MPEG-1 Audio, published under ISO-IEC/JTC1 SC29 11172-3, entitled “Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to About 1.5 Mbits”, and the disclosure of that document is incorporated herein by reference. MPEG-2 Audio (ISO/IEC 13818-3) provides the extension to 3/2 multichannel audio and an optional low frequency enhancement channel (LFE). The audio part of the standard, ISO/IEC 11172-3, defines three algorithms, Layer 1, 2 and 3 for coding PCM audio signals. MPEG-2 (Multichannel) also defines Layer 1, 2, and 3 algorithms.
The MPEG audio encoder processes a digital audio signal and produces a compressed bitstream for transmission or storage. The encoder algorithm is not standardised, and may use various means for encoding such as estimation of the auditory masking threshold, quantisation, and scaling. However, the encoder output must be such that a decoder conforming to the above-mentioned standards specification will produce audio suitable for the intended application.
The decoder, subject to the application-dependent parameters, accepts the compressed audio bitstream in the defined syntax, decodes the data elements and uses the information to produce digital audio output, also according to the defined standard. The decoder first unpacks the received bitstream to recover the encoded audio information frame by frame.
After the process of frame unpacking, the decoder performs an inverse quantisation (expansion process) and feeds a sub-band synthesis filter bank with a set of 32 scaled-up sub-band samples in order to reconstruct the output PCM audio signals. The sub-band filter banks used for Layer 1 and Layer 2 of MPEG 1 audio decoder and Layer 1 and Layer 2 of MPEG2 (Multichannel extension) audio decoder, are the same.
The sub-band synthesis filter is one of the most computationally intensive blocks of the MPEG audio decoder. Sub-band filtering is performed for each sub-band in a frame and for every channel. Any reduction in its computational requirements thus enables less complexity and reduced cost of decoding.